55 
is t-distributed with approximately 220 degrees of freedom. With this large 
number of degrees of freedom it is almost normally distributed with mean 0 
and standard deviation 1. These quantities are determined for different radii 
and used to test the equality of the curves. They turn out to be different. The 
calculations are shown in Tables 9 and 10. 
The co-ordinates of the principal points, their standard errors, and tests of 
the variation between the pictures are given in Table 11. 
The discussion above indicates that the interior orientation cannot be assu 
med constant. The variation between the determinations is highly significantly 
different from the separate standard errors. 
8.2.8. Relative Orientation and Base 
The exterior orientation elements are now transformed to a co-ordinate sys 
tem having its X-axis along the base, its origin in one of the exterior projection 
centers, and its X—Z-plane containing the corresponding camera axis. 
Firstly the base components are computed as the differences of the co-ordi 
nates of the two exterior projection centers. They are denoted by BX, BY, BZ. 
Fig. 15. 
Stereo-Medical-Nikkor. Radial distortion curves transformed to the same camera constant, 
256 mm. Left diagram: camera 1. Right diagram: camera 2. Curves 1 and 2: f-stop 45. Curves 3: 
f-stop 11.